The present invention is directed generally to optical transmission systems. More particularly, the invention is directed toward optical transmission systems including optical amplifiers configured to provide for in-service pump maintenance, replacement, and upgrades.
Digital technology has provided electronic access to vast amounts of information. The increased access has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems to link the processing equipment.
In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information bandwidth transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal formats analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals into a higher bit rate signals that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal can then be separated from the higher bit rate signal following transmission through the optical system.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, i.e., far-UV to far-infrared. The pluralities of information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration/amplification equipment required to compensate for signal attenuation for each optical wavelength throughout the system. The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically regenerate attenuated optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical regenerators.
The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal regeneration/amplification equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.
The increased capacity of information transmitted in WDM systems has made system downtime increasingly more expensive in terms of lost capacity and revenue. Furthermore, the amount of excess system capacity must be increased accordingly to ensure a continued quality of service to customers in the event of a shutdown in part or all of the optical system.
While EDFAs have contributed greatly to the development of WDM systems, EDFAs are a source of failure that can result in the shutdown of an optical link in the system. More specifically, lasers and other optical sources, used in pump sources to supply optical energy to an erbium doped fiber in an EDFA, have proven to be a common point of failure in optically pumped amplifiers.
A pump source failure generally causes an amplifier and optical link shutdown resulting in lost revenues, degraded service quality, and unscheduled labor costs. Because of the high cost of amplifier shutdowns, pump sources generally include redundant optical sources to improve the reliability of the amplifiers.
The need for and use of redundant optical sources does reduce the power requirements and cost, as well as increase the reliability, of the optical sources. In various EDFA designs, multiple optical sources are combined and used to pump multiple fiber amplifiers to provide redundancy and to more cost effectively deploy the optical sources. For example, see U.S. Pat. Nos. 4,699,452, 5,039,199, and 5,173,957.
The use of redundant optical sources does not improve the reliability of the optical sources, but only the reliability of the optical amplifier. For example, the failure of an optical source in amplifiers having redundant optical sources will generally not cause an unplanned amplifier shutdown, but will usually require a planned maintenance shutdown to repair or replace the failed optical source. A planned maintenance shutdown allows communications traffic to be rerouted to provide continued service and work scheduled, which greatly reduces the cost of the maintenance. Nonetheless, traffic must still be rerouted and the link taken out of service to repair the failed or degraded pump sources resulting in lost revenue and increased costs.
The provisioning of optical systems and the traffic planning costs associated with rerouting of traffic imposes a substantial financial burden on communications service providers. Rerouting of traffic is a particular concern in optical systems that do not have a highly connected network or significant excess capacity to reroute traffic.
Given the trend of continually increasing communication traffic is not expected to diminish, the need for more reliable, higher capacity optical systems will continue to grow. Thus, it is imperative that optical systems be developed including optical amplifiers with increased reliability to allow the optical systems to be operated in an efficient, cost-effective manner.